The discovery of a topological insulator in 2005 led to remarkable development of topological band theory, revealing a variety of symmetry-protected topological insulators and semimetals. Here we introduce our recent finding of a novel topological crystalline insulating phase, referred to as a topological Dirac insulator [1]. A topological Dirac insulator is a bulk insulator with protected metallic surface states, allowed by non-symmorphic space group symmetries. Unlike conventional topological insulators, the surface states of a topological Dirac insulator occur as a four-fold degenerate Dirac point, considered as a topological phase boundary between a two-dimensional topological insulator and a normal insulator. We introduce Z4xZ2 topological invariants that characterizes topological Dirac insulator phase and demonstrate how to evaluate from the Wilson loop calculations. We also discuss its material realizations based on first-principles calculations.